1. Field of the Invention
This invention relates to an antireflection optical coating suitable for optical parts of relatively low refractive index, particularly for optical parts made of synthetic resin.
2. Description of the Prior Art
While recently, optical parts made of synthetic resin have been gradually used due to their ease of processing and lightweight, a problem still remains with respect to the antireflection properties of these optical parts.
For optical parts made of synthetic resin, particularly for lenses, silicon oxide is widely used as one of evaporation materials for the coating film. This results from the fact that the silicon oxide evaporation film relatively firmly adheres to the synthetic resin surface, and a hard film can be obtained without heating the resin part under vacuum.
In a well known optical coating using silicon oxide, silicon dioxide (SiO.sub.2) film having the refractive index 1.46-1.48 is used as a first layer which is in contact with air. The bulk refractive index of SiO.sub.2 is about 1.46 but the refractive index of the evaporation film is generally higher than the bulk refractive index due to the thermal decomposition during the course of evaporation. On the other hand, when the evaporation slowly takes place under an atmosphere of oxygen of about 10.sup.-4 Torr, using an electron gun for heating an evaporant, the refractive index of the evaporation film can come closer to the bulk refractive index. However, even if the refractive index of the evaporation film becomes the same as the bulk refractive index, it is not possible to achieve a sufficient antireflection effect as long as a single layer of SiO.sub.2 is used.
Generally, in the antireflection films, the amplitude condition cannot be satisfied unless the refractive index n.sub.1 of the first layer film in contact with air is smaller than the refractive index n.sub.s of a substrate. For example, in a single layer antireflection coating (n.sub.0/4), in case n.sub.1 =n.sub.o .sqroot.n.sub.s (where, n.sub.o is the refractive index of refraction of air) is satisfied, the reflectivity is zero at the center wavelength (No).
Synthetic resins used for optical parts are transparent resins such as polymethyl methacrylate (PMMA) n=1.49, diallyl glycol carbonate (CR39) n=1.50, polystyrene (PS) n=1.59, acrylonitrile-styrene (AS) n=1.57, polycarbonate (PC) n=1.58, etc., which however, cannot satisfy the aforesaid amplitude condition with the refractive index of SiO.sub.2. Particularly, with respect to the resins such as PMMA, CR39, etc. of low refractive index of refraction which are often used, the SiO.sub.2 single layer coating makes it hardly possible to attain the desired antireflection effect.
For the above reasons, if an attempt is made to obtain a multi-layer film more of than two layers, more than two kinds of evaporation materials must be used, and a plurality of evaporation sources must be set within an evaporation tank and, in addition, if SiO.sub.2 is used, an electron gun must be used as a source of evaporation, thus resulting in a complicated process for manufacture, which results in high costs.